Back to work

It’s been great having time off work with Karen and Benjamin, but, as they say, all good things come to an end. So today it’s back to work, with my first class in about an hour and a half. I spent my final afternoon of parental leave probing the underlying geology of our driveway, trying to work out why it was flooding in patches. (The fact that it had deluged for two days pretty-well non-stop was clearly half the reason, but why wasn’t the water clearing?) As it turned out, the gravel top surface was hiding sizeable potholes underneath, which were just filling with water, which wasn’t draining. When you’ve got time to do that, you know that you don’t need to be on parental leave anymore!

But that’s not the subject of today’s blog. It’s Microwave sterilizers. They are a handy device for rapid sterilization of bottles and other baby parephanalia. Fill with 200 ml of water, zap on full power (in our case 900 W) for 5 minutes, and it’s done.

A quick physics calculation will show what’s happening. A 900 watt microwave will put out a total of 270 thousand joules of energy in five minutes (that’s 900 joules per second times 300 seconds). A paltry 4.2 joules will take 1 ml of water up in temperature by 1 degree C (that’s the specific heat capacity). So to take 200 ml of water from the tap (at say 15 C) and take it to 100 C (boiling) requires 85 times 200 times 4.2 equals 71 400 J of energy. That’s about a quarter of what is being supplied.

So the water will reach boiling point. But where does the extra energy go (there’s about 199 thousand joules of it)? This is put into turning the water at 100 C into steam at 100 C. It requires a considerable energy input to make this step. The latent heat of vaporization of water is about 2300 joules per gram (or ml of water). This means that the extra 199 thousand joules will turn about 80 or so ml into steam. That means there should be plenty of water left at the end of the proceedings.

Of course, the steam will start condensing on the bottles and internal surface of the sterilizer – these have a specific heat capacity too and will take some energy to come up to a similar temperature. But the rough calculation above I hope shows that the 5 minutes for 900 W statement by the manufacturer is pretty sensible.

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